1. Technical Field
This invention relates to a telephone communications system and, in particular, to a method of increasing a reception count in a case of constructing a multifrequency signal receiver using the Goertzel algorithm by a digital signal processor (DSP).
2. Background Art
As is well known in the art, there are a dual tone multi-frequency (DTMF) signal and a multi-frequency code (MF) signal as a multifrequency signal (a selection signal). The DTMF signal is a signal obtained by combining two frequencies (one selected from a low frequency group and one selected from a high frequency group) in a speech frequency band. The low frequency group consists of four frequencies of 697 Hz, 770 Hz, 852 Hz, and 941 Hz. The high frequency group consists of four frequencies of 1209 Hz, 1366 Hz, 1477 Hz, and 1633 Hz. On the other hand, the MF signal consists of a selection number by pulses obtained by combining two frequencies selected from six frequencies of 700 Hz, 900 Hz, 1100 Hz, 1300 Hz, 1500 Hz, and 1700 Hz. Accordingly, the DTMF signal and the MF signal have the maximum frequency of 1633 Hz and 1700 Hz, respectively.
Throughout the instant specification, the DTMF signal and the MF signal are collectively called the multifrequency signal. That is, the multifrequency signal comprises the DTMF signal and/or the MF signal.
In a multifrequency signal receiver for receiving such a multifrequency signal, DFT (Discrete Fourier Transform) for each of the prescribed frequencies with a method that uses the Goertzel algorithm is carried out and strength of each frequency component is detected. Herein, the Goertzel algorithm is an algorithm which performs DFT similarly to a FFT (Fast Fourier Transform) algorithm, and is advantageously used when only a limited number of frequency components are to be detected (for example, see Japanese Unexamined Patent Application Publication of Tokkai No. 2000-324519 or JP-A2000-324519 (which will be also called Patent Document 1), which corresponds to U.S. Pat. No. 6,731,745).
In a telephone communications system, it is necessary to increase a reception count in the multifrequency signal receiver. However, it is difficult costwise to improve a spec of the DSP (digital signal processor) for carrying out a receiver processing.
Various prior art documents related to the present invention are known. By way of example, Japanese Unexamined Patent Application Publication of Tokkai No. Hei 2-69093 or JP-A 2-69093 (which will be also called Patent Document 2) discloses an technical idea for simultaneously receiving, in one multi-frequency receiver, multifrequency signals from a plurality of subscribers by periodically switching the connection of channel paths therebetween.
Patent Document 2 merely discloses a time division technique which recognizes a timing of an ON/OFF time of the DTMF signal (the multifrequency signal) and uses an OFF time for detection processing of other DTMP signals (multifrequency signals).
Japanese Examined Patent Application Publication of Tokou No. Hei 6-66980 or JP-B 6-66980 (which will be also called Patent Document 3), which corresponds to U.S. Pat. No. 4,604,713, discloses a multi-frequency receiver including an ADC (analog-to-digital converter) for sampling an input signal at 8 KHz and converting to a digital signal and a low-pass filter (LPF) having a frequency bandwidth ranging from 0 to 2 KHz. In Patent Document 3, an output signal of the LPF is resampled at the frequency of 4 KHz before being fed into first and second Hilbert transformer type filters. The first Hilbert transformer type filter is a band-pass filter supplying the in-phase and quadrature components of the signals of the so-called low-frequency groups (627 Hz; 770 Hz; 852 Hz and 941 Hz). The second Hilbert transformer type filter is a band-pass filter supplying the in-phase and qudrature components of the signals of the so-called high-frequency groups (1209 Hz; 1336 Hz; 1477 Hz and 1633 Hz). A first polar coordinates converter processes an output signal of the first Hilbert transformer type filter to convert signals from Cartesian coordinates into polar coordinates. A second polar coordinates converter processes an output signal of the second Hilbert transformer type filter to convert signals from Cartesian coordinates into polar coordinates.
A tone detector disclosed in Patent Document 3 uses a sampling frequency of 8 kHz in a telephone system. In Patent Document 3, data supplied to the detector at the same sampling time interval is used as sampling data without time division.